Variable speed wind turbine generator

ABSTRACT

A variable speed system for use in systems, such as, for example, wind turbines, is described. The system comprises a wound rotor induction generator, a torque controller and a proportional, integral derivative (PID) pitch controller. The torque controller controls generator torque using field oriented control, and the PID controller performs pitch regulation based on generator rotor speed.

This application is a divisional of application Ser. No. 09/640,503,entitled “VARIABLE SPEED WIND TURBINE GENERATOR”, filed Aug. 16, 2000,which is a divisional of application Ser. No. 08/907,513, entitled“VARIABLE SPEED WIND TURBINE GENERATOR”, filed Aug. 8, 1997 now U.S.Pat. No. 6,137,187.

FIELD OF THE INVENTION

The present invention relates to the field of wind turbines; moreparticularly, the present invention relates to variable speed windturbines having a doubly fed generator and applying torque control andpitch regulation based on generator rotor speed.

BACKGROUND OF THE INVENTION

Recently, wind turbines have received increased attention asenvironmentally safe and relatively inexpensive alternative energysources. With this growing interest, considerable efforts have been madeto develop wind turbines that are reliable and efficient.

Generally, a wind turbine includes a rotor having multiple blades. Therotor is mounted within a housing, which is positioned on top of a trussor tubular tower. The turbine's blades transform wind energy into arotational torque or force that drives one or more generators,rotationally coupled to the rotor through a gearbox. The gearbox stepsup the inherently low rotational speed of the turbine rotor for thegenerator to efficiently convert mechanical energy to electrical energy,which is fed into a utility grid.

Many types of generators have been used in wind turbines. At least oneprior art wind turbine has included a doubly-fed wound rotor generator.See U.S. Pat. No. 4,994,684, entitled “Doubly Fed Generator VariableSpeed Generation Control System;” issued Feb. 19, 1991.

A wound rotor induction generator (WRIG) typically includes four majorparts: the stator, the rotor, slip rings, and the end caps withbearings. A cross-sectional view of a two-pole 3-phase generator isshown in FIG. 1 where, for simplicity, the windings are shown as a pairof conductors. Referring to FIG. 1, generator 100 comprises stator 101,rotor 102, and winding phase A for each of the rotor and stator, 103 and104 respectively. A shaft 105 that couples the blades of the windturbine trough the gear box to generator 100 is also shown.

Referring to FIG. 2, in a WRIG system, stator winding 104 is typicallyconnected to the 3-phase utility power grid, such as 480V, 3-phase grid201, and the rotor winding 103 is connected to a generator-side inverter202 via slip rings (not shown). The winding 104 is also coupled to the480V, 3 phase source 201 in parallel with a line-side inverter 203. Theline-side inverter 203 and generator-side inverter 202 are coupledtogether by DC bus 204. The configuration shown in FIG. 2 (i.e.,line-side inverter 203, DC bus 204, and generator-side inverter 202)allows power flow into or out of the rotor winding 103. Both invertersare under the control of a digital signal processor (DSP) 205.

Many conventional wind turbines rotate at a constant speed to produceelectricity at a constant frequency, e.g., sixty cycles per second (60Hz), which is a U.S. standard for alternating current or at 50 Hz whichis a European standard. Because wind speeds change continuously, thesewind turbines utilize either active (pitch regulation) or passive (stallregulation) aerodynamic control in combination with the characteristicsof conventional squirrel cage induction generators for maintaining aconstant turbine rotor speed.

Some turbines operate at variable speed by using a power converts toadjust their output. As the speed of the turbine rotor fluctuates, thefrequency of the alternating current flowing from the generator alsovaries. The power converter, positioned between the generator and thegrid, transforms the variable-frequency alternating current to direcurrent, and then converts it back to alternating current having aconstant frequency. The total power output of the generator is combinedby the converter (total conversion). For an example of such a turbine,see U.S. Pat. No. 5,083,039, entitled “Variable Speed Wind Turbine”,issued Jan. 21, 1992.

Using variable speed wind turbines to generate electrical power has manyadvantages that include higher propeller efficiency than constant speedwind turbines, control of reactive power—VARs and power factor, andmitigation of loads.

Some prior art variable speed wind turbines are total conversion systemsthat use a power converter to completely rectify the entire power outputof the wind turbine. That is, the wind turbine, operating at a variablefrequency, generates a variable frequency output and converts it into afixed frequency for tracking the grid. Such systems that utilize totalconversion are very costly. Because of the cost, parties are oftenseeking lower cost solutions, such as for example, a wound rotorgenerator system utilizing partial conversion in which only a portion ofthe wind turbine output is rectified and inverted by the powerconverter.

Some problems currently exist with various control algorithms used bythe power converters to control the partial conversion process. Forinstance, certain systems have stability problems in that they havelarge oscillations in power and torque. Other systems cannot produceenough power without overheating critical components or are not easilyrefined to provide a cost effective solution for series production.

Thus, a need exists for a low cost wind turbine system that does nothave the stability problems of the prior art, yet still produces a largeamount of power, cost effectively, without generating excessive amountsof heat and can be easily refined into a cost effective, readilyproducible design.

SUMMARY OF THE INVENTION

A variable speed system for use in systems, such as, for example, windturbines, is described. The system comprises a wound rotor inductiongenerator, a torque controller and a pitch controller. The torquecontroller controls generator torque using a field orientation controlapproach. The pitch controller performs pitch regulation based ongenerator rotor speed which is independent of the torque controller.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the invention, which, however, should not be taken tolimit the invention to the specific embodiments, but are for explanationand understanding only.

FIG. 1 illustrates a cross sectional view of a simplified wound rotorinduction generator.

FIG. 2 illustrates a typical system configuration incorporating a woundrotor induction generator.

FIG. 3 illustrates the equality relationship between torque and thecross product of current and flux.

FIG. 4 illustrates a wound field DC motor.

FIG. 5 illustrates flux direction when only “A” phase is energized.

FIG. 6A is a flow diagram of one embodiment of the system of the presentinvention.

FIG. 6B is a block diagram of one embodiment of the wound rotorinduction generator and torque control of the present invention.

FIG. 6C illustrates the relation between flux vector and rotor currentvector.

FIG. 6D illustrates components of the rotor current

FIG. 7 is a flow diagram of one embodiment of the wind turbinecontroller of the present invention illustrating the enable/disablesequence for the power/torque controller and the different modes of thepitch controller.

FIG. 8 is a flow diagram of one embodiment of the pitch regulation modeof the present invention.

FIG. 9 is a flow diagram of one embodiment of the rpm regulation mode ofthe present invention.

FIG. 10A is a block diagram of one embodiment of a pitch control system.

FIG. 10B is a block diagram of one embodiment of the proportional,integral, derivative (PID) pitch controller of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A variable speed system is described. In the following description,numerous details are set forth, such as set points, numbers of watts,etc. It will be apparent, however, to one skilled in the art, that thepresent invention may be practiced without these specific details. Inother instances, well-known structures and devices are shown in blockdiagram form, rather than in detail, in order to avoid obscuring thepresent invention.

Overview of the Present Invention

The present invention provides a variable speed system. In oneembodiment, the variable speed system comprises a wind turbine generatorwith power/torque capability, which is coupled to and supplies generatedpower to a grid. In one embodiment, the generator of the presentinvention comprises a wound rotor induction generator (WRIG or doublyfed generator (DFG)) and a rotor that utilizes blade pitch regulationand variable speed operation to achieve optimum power output at all windspeeds.

An induction generator's ability to generate power is equivalent to itsability to produce torque at rotational speed. When a torque is exertedon the generator's rotor in the opposite direction of its rotation, themechanical energy of the rotor is converted to electrical energy. In aninduction generator, torque is derived from the interaction betweencurrent and flux as illustrated in FIG. 3 or, more precisely, torque isthe cross product of current and flux. To obtain maximum torque for agiven flux level, the direction of the rotor current vector is keptexactly at 90 degrees from the direction of the flux. In a DC motor,this perpendicular relationship between flux and armature current isaccomplished by commutators.

FIG. 4 shows the mechanical structure of a wound field DC motor. Becauseof the separate armature and field windings, a DC motor can becontrolled by regulating the armature current for a desired torqueoutput and by regulating the field winding current for the desired fluxintensity.

Torque generation in an induction generator operates on the sameprinciple as in a DC motor. The main difference between the two is that,in a DC motor, both the flux and armature current direction are fixed,while in a induction generator, these two vectors are constantlyrotating.

Field oriented control (FOC) is an algorithm that identifies the fluxvector and controls the torque production current accordingly.

FIG. 5 shows the flux direction when only stator winding phase A isenergized. In the system shown in FIG. 2, stator phases are energizedsequentially by a 3-phase voltage source and this creates a rotatingflux vector.

Note that both flux and the 3-phase current are two-dimensional (2D)vectors (i.e., with a magnitude and an angle), and with zero rotorcurrent, flux vector (Ψ) is related to stator current vector (Is) by thefollowing algebraic equation:

Ψ=Ls*Is  (1)

where Ls is the stator inductance. Without the rotor winding beingenergized, the generator behaves like an inductor, i.e. the statorcurrent lags stator voltage by 90 degrees or, more precisely,$\begin{matrix}{{Vs} = {{\frac{}{t}\Psi} = {{Ls}\frac{{Is}}{t}}}} & (2)\end{matrix}$

where Vs represents the stator voltage.

An important element in the FOC is the flux model. The flux model isused to identify the flux vector. Equation (1) is a very simple form offlux model for a WRIG and indicates that the flux vector can be simplyidentified by taking the product of stator current measurement (Is) andthe stator inductance (Ls). By using the flux model, the flux vector maybe identified so that torque may be controlled to generate power.

Although the following discussion describes the present invention interms of a variable wind speed turbine, the present invention hasapplication to other electrical and mechanical systems. For instance,the generator of the present invention may be used in systems that haveother sources that rotate a shaft coupled to the generator rotor, suchas hydroelectric, gas turbine, and general prime mover systems, etc.

In one embodiment, the wind turbine of the present invention comprises arotor which is 3-bladed and comprises blades with full span blade pitchcontrol, pitch bearings, and a hub. Such a wind turbine rotor iswell-known in the art. Note that any number of blades or any turbineconfiguration may be used in the present invention. The wind turbinerotor is coupled to an integrated drive train that includes a mainshaft. The main shaft is coupled to the generator.

The system of the present invention also comprises a power converter inthe excitation circuit between the grid and the wound rotor of thedoubly fed wound rotor induction generator. The stator is coupled, via acontactor, to the grid. Since the converter is in the rotor circuit, itprocess (e.g., converts) a fraction of the total rated kilowatt (KW)output of the turbine rotor. In one embodiment, the total rated outputof the turbine rotor comprises 750 KW, and the converter converts atmost 25-30 percent of the total rated power (e.g., 160 KW). In oneembodiment, the generator comprises a 750 KW, 460 volt, wound rotorinduction generator.

In one embodiment, the present invention provides a variable speedsystem having a wound rotor induction generator, a torque controller,and a proportional, integral derivative (PID) pitch (or speed)controller. The induction generator of the preset invention may comprisea slip ring or non-slip ring induction generator. The variable speedsystem uses the wound rotor induction generator with a power convertersystem to ensure the delivery of constant frequency power to the grid.Note that although grid applications are described, it would be apparentto one skilled in the art that the present invention may also be appliedto other applications such as stand-alone power systems.

The torque controller, which is typically part of a power converter,controls the torque of the generator. In one embodiment, the torquecontroller controls generator torque as a function of generator speedwith a field oriented control (FOC) approach using flux vector controlThe torque controller operates on the generator from cut-in to ratedwind speeds. In one embodiment, cut-in refers to the lowest wind speedat which the generator or turbine is designed to operate, while ratedspeed is the minimum wind speed at which the turbine produces itsmaximum power (e.g., 750 kw). In one embodiment, at above rated windspeeds, the torque controller holds the generator rotor at a constantpower.

In one embodiment, the power controller comprises a look up table (LUT)that outputs power values as a function of generator rotor speeds. Thepower controller interpolates the LUT, which contains an encodedpower-speed curve, to obtain a target output power. This power is thendivided by the measured generator rotor speed to obtain a desiredgenerator torque from the equation T=P/ω (torque=power/angularvelocity). In one embodiment, the output of the LUT is a target outputpower, which is compared to the actual output power, using comparator ordifferencing hardware or software, to generate a power error indication.A proportional, integral (PI) controller generates an adjusted actualoutput power value in response to the power error indication, which whendivided by the measured generator rotor speed, by divider hardware orsoftware, results in a commanded torque. The commanded torque causes aspecified rotor current vector to be impressed onto the rotor, whichinteracts with an identified flux vector to produce a desired generatortorque.

Thus, the present invention also provides for controlling generatortorque by measuring actual generator rotor speed, accessing a LUT usingmeasured rotor speed to obtain a target output power, comparing actualoutput power to the target output power, and generating a commandedtorque by adjusting a torque calculation to maintain a predeterminedoutput based on the comparison of actual output power to the targetoutput power.

In one embodiment, a process for synchronizing such a variable speedsystem is used that includes connecting a generator stator, connecting agenerator rotor, ramping up a rotor current magnetizing current Ird(torque producing component of the rotor current) and regulatinggenerator torque by controlling the flux producing component of therotor current Irq.

The system of the present invention also includes a variable pitch andspeed regulation subsystem which provides real-time proportional pitchposition, as well as turbine speed regulation, by using a proportional,integral, derivative (PID) controller.

The PID controller performs pitch regulation based on generator rotorspeed and operates independently of the torque controller in the powerconverter. In one embodiment, the PID controller is a dosed loop PIDcontroller that generates a pitch velocity to perform pitch regulationwhile at or above rated wind speeds. In one embodiment, the PDcontroller may begin to perform pitch regulation during less than aboverated wind speeds. In one embodiment, below rated speed, the pitch angleis fixed at full power-on position.

The PD controller controls generator rotor speed by pitching the bladesof a wind turbine. In one embodiment, the PID controller generates anoutput voltage in response to a difference between the target rotorspeed and the measured (or actual) rotor speed, which a non-linear LUT(in one embodiment, table 1011 of FIG. 10) uses to output a pitchvelocity in response thereto.

Although the present invention is described in conjunction with a PIDcontroller, a proportional, integral (P) controller, a proportional,derivative (PD) controller, or a proportional controller may be used inembodiments. Other lead-lag or lag-lead controllers can also be used.Also, although the present invention is described in conjunction with aclosed loop controller, an open loop controller may be used, such as anopen loop controller with a derivative term. These types of controllersare well-known in the art.

System Overview

FIG. 6A illustrates one embodiment of a system according to the presentinvention. Referring to FIG. 6A, a generator torque control 603 in avariable speed converter is coupled to receive a calculated torque 601based on measured rpm 607 and a preselected maximum torque set point602. In one embodiment, calculated torque 601 is a function of measuredrpm of the generator based on look up table/power-speed curve 640. Theoutput of table 640 is divided by the measured rpm 607 using divider641.

In one embodiment, the maximum torque 602 is set at approximately 5250Nm and its selection is based on maximum current available fromconverter system thermal ratings. In other words, the selection is basedon a calculated torque speed characteristic curve for a particularturbine rotor designs. In one embodiment, this selection is based on anexcitation amount of 290 amps.

In response to these inputs, torque control 603 generates a torquecommand to control the generator rotor 604. Torque control 603 is alsocoupled to receive a VAR or power factor command 642.

Generator rotor 604 is coupled to receive the torque command fromgenerator torque control 603 and is coupled to provide power via a fluxgap to generator stator output 605. A feedback 612 is coupled fromgenerator stator output 605 to the input of generator rotor 604. Theoutputs of generator rotor 604 and generator stator 605 are coupled toutility grid 606.

Generator rotor 604 is also coupled to a measuring device which producesa measured speed 607 (in rpm) of generator rotor 604. In one embodiment,the measuring device comprises an optical encoder that provides positionas well as rotational speed of generator rotor 604.

A proportional, integral, derivative (PID) controller and pitch ratelimit block 609 is coupled to receive the measured speed 607 and anoperating speed (rpm) set point 608. The operating speed set point maybe set based on the same torque speed characteristic used to establishthe maximum torque setpoint. In one embodiment, the operating speed setpoint is based on maximum torque and power. In one embodiment, theoperating speed set point 608 is 1423 rpm. In response to these inputs,the PID and pitch rate limit block 609 generates a voltage output.

A variable pitch control (VPC) 610 is coupled to receive the pitchvelocity output from PID and pitch rate limit block 609. VPC 610 iscoupled to blade rotor 611 to regulate the speed of generator rotor 604by controlling the input aerodynamic torque of blade rotor 611 throughblade pitch action. PID and pitch rate limit block 609 generates adesired pitch velocity which is converted to a voltage using a table, asdescribed in more detail below. A variable voltage output is applied toa proportional value in a hydraulic system that pitches blades byactuating a pitch cyclinder at variable rate. Thus, the variable pitchcontrol regulates the rpm by controlling aerodynamic torque.

The PID and pitch rate limit block 609 including the measured rpm 607and the operating speed (rpm) set point 608, VPC 610 and the blade rotor611 form blade pitch system 650, while the measured rpm 607 and theremaining portion of the system in FIG. 6A are part of power converterand generator system 651. Note that in one embodiment the measured rpm607 is used simultaneously by the blade pitch system 650 and the powerconverter/generator system 651.

The Power Converter of the Present Invention

In the present invention, the power converter controls the wound rotorinduction generator according to a predetermined power-speed curve. Byfollowing the predetermined power-speed curve, the variable speed systemis able to operate the turbine at the maximum power coefficient (Cp)from cut-in to rated wind speeds, which is referred to herein as RegionII, thereby ensuring that maximum aerodynamic energy capture isachieved. It should be noted that the power-speed curve is related to atorque-speed curve since P=Tω.

In one embodiment, the power-speed curve is encoded in the powerconverter in the form of a look-up table (LUT) of power andcorresponding generator speeds. The LUT may reside in hardware orsoftware. To control torque, the power converter measures generatorrotor speed, interpolates the LUT to determine the target turbine outputpower, and calculates the desired generator torque from the relationT=P/ω using the generator rotor speed. In one embodiment, this torque isproduced by determining the required current vector and, usingwell-known pulse width modulation techniques, produces this vector.

In one embodiment, due to slight differences between theoretical andactuality, the power converter of the present invention employs a closedloop PI controller which compares actual turbine power output to atarget, or desired, output and makes small adjustments to the torquecalculation to achieve and maintain a desired turbine output.

The torque controller of the power converter uses field oriented control(FOC) to produce generator torque as a function of generator rotorspeed. Using the stator current, the rotor current and the rotor angleas inputs, the torque controller of the power converter identifies theflux vector and commands the required rotor current vector which, uponinteraction with the stator flux vector, produces the desired generatortorque. The rotor current is created by the appropriate switching of theconverter insulated gate bipolar transistors (IGBTs) using well-knownpulse width modulation (PWM) current regulation techniques, such asdescribed in U.S. Pat. No. 5,083,039, entitled “Variable Speed WindTurbine”, issued in Jan. 21, 1992. In this manner, the power controlsystem follows an aerodynamically optimized power/torquespeed profile.

Note that the look up table values containing the power/torque-speedprofile are based on the aerodynamics of the particular wind turbinerotor and wind turbine rotor geometry. Therefore, the table values setmay vary for different turbine rotors.

One embodiment of the torque controller and pertinent portions of thewound rotor induction generator are shown in FIG. 6B. Torque can beexpressed as

Td=k*Ψ*Irq  (3)

where k is a generator parameter. From the controller point of view,equation (3) takes the following form.

Irq=Td/(k*Ψ)  (4)

Equation (4) gives the magnitude of the rotor current for a given‘desired torque’ Td, which is output from torque command controller 623.

Referring to FIG. 6B, the torque controller 623 comprises a power table623A, PI controller 623B, divider 623C, switch 629 and comparators 623Dand 623E, which may be implemented in hardware or software to generatedifference values, and a feedforward dampening filter 623F. Power table623A is a LUT coupled to receive the generator speed 607 and outputs atarget power value corresponding to the generator speed 607. Oneembodiment of power table 623A is shown in Table 1 below.

TABLE 1 Generator Electrical Speed Power RPM (kW)  750 −177   800 −177  850 135  900 167  950 203 1000 247 1050 287 1100 335 1150 388 1200 4501250 507 1300 575 1350 647 1400 743 1450 750 1500 750 1550 750 1600 750

The target output power is compared by comparator 623D to generate adifference between the target output power and the actual output power.The resulting difference is input to PI controller 623B which adjuststhe power as described herein. Divider 623C is coupled to receive theadjusted power from PI controller 623B and generator speed 607 to outputa commanded torque.

The commanded torque may be adjusted by a torque value generated byfeedforward dampening filter 623R. Dampening filter 623F detectsoscillation motion (at resonance) of the non-rigid (compliant) shaft(not shown to avoid obscuring the invention) caused by its couplingbetween two separate inertias, one due to the gear box and generator andthe other due to the blades of the turbine. In response to thisdetection, dampening filter 623F applies a negative torque to reduce therelative motion between the two inertias. In one embodiment, dampeningfilter 623F comprises a bandpass filter in which the passband iscentered at the resonant frequency of the two inertias and the shaft.

The resulting commanded torque is impressed upon the wound rotor of theinduction generator.

Switch 629 operates in response to a braking indication (e.g.,signal(s)) to switch the commanded torque to a maximum constant torque660, as described in more detail below.

For torque production operation, a rotor current component Irq iscontrolled to follow the perpendicular direction of the flux (see FIG.(6D)). The magnitude of Irq is given by the equation below.

Irq=Td/(k*Ψ)

where k is a generator parameter. Note that rotor current, Ird, which isdiscuss in more detail below, creates the generator flux and does notcontribute to torque production.

Rotor current component block 622 is coupled to receive the commandedtorque and the scalar component of the flux vector fromrectangular-to-polar coordinate transform block 626, which converts theflux vector from flux model 621 into polar coordinates. In response tothese inputs, rotor current component block 622 generates the rotorcurrent torque component, Irq.

Flux model 621 identifies the flux vector. To identify the flux vector,current converter blocks 621A and 621B obtain the stator current vectorand the rotor current vector. Note that since the current vector can bedetermined from measuring two of the three phase current, only twocurrent sensors (not shown) are required. The stator current vector withrotor angle 621B of generator 620 are input to frame transform block627C. Frame transform block 627C transforms the stator current to arotor fixed frame. From outputs of frame transform block 627C, thestator inductance Ls is determined at block 621D. From the rotor currentvector, the rotor inductance Lr may be obtained at block 621F. The fluxvector is generated from the stator inductance Ls and the rotorinductance Lr.

Once the flux vector is determined, the rotor current vector output frominverter 624 is “positioned” in the perpendicular direction of the fluxso as to produce torque. Since rotor current is specified with respectto rotor assembly, the rotor current command depends on both flux angleand rotor angle. Specifically, the flux angle is first transformed intoa rotor fixed reference frame and, in this frame of reference, thedirection of the rotor current command is the direction that isperpendicular to the flux direction. This procedure is shown in FIG. 6C.

Using the rotor current component, Irq, in conjunction with theinductive portion of the output of transform block 626, a currentreference is generated at the input of inverter 624. Also shown isinverter 630 coupled to inverter 624 via DC bus 631 and coupled to thestator side (line side) of generator 620.

When this rotor current is forced to flow through the rotor windings,the desired torque Td is developed and the power (Td*ω) is generatedwhere ω is the rotor speed. This power is generated in the form ofstator current that flows back into the grid. This ‘power carrying’stator current is in-phase with the stator voltage.

When power is being produced by the generator, the flux model describedin Equation (1) above is no longer valid since the stator current (Is)now consists of two components: flux producing component and the powercarrying component This power carrying component does not contribute toflux production because this current component has the same magnitude(after normalized by winding ratio) as the torque producing rotorcurrent but in the opposite direction. In other words, flux produced bythese two current vectors (ie., power carrying stator current and torqueproducing rotor current) summed together is zero. To remove the powercarrying component from the stator current measurement, the rotorcurrent (Ir) is added to Equation 1 above, i.e.,

Ψ=Ls*Is+Lr*Ir

where Lr is the rotor inductance. Ls and Lr differ by winding ratio.

Note that in the operation described above, while the power carryingstator current component is in phase with the stator voltage, the fluxproducing component lags stator voltage by 90°. This flux producingcurrent component results in non-unity stator power factor. Since fluxproducing current inherently lags voltage by 90°, to achieve unity powerfactor on the stator side, the flux is produced by the rotor winding.

To produce flux by rotor winding, an additional component, Ird, of rotorcurrent should be commanded. This additional component should be alongthe direction of flux as shown in FIG. 6D.

As the flux producing component of the rotor current (Ird) increases,the flux producing stator current decreases. This is due to the factthat the flux magnitude is kept constant by the constant stator voltage(from Equation 2 above). The flux producing component of the rotorcurrent, Ird, can be controlled in such a way that the flux it producesinduces the same voltage as the grid voltage. That is, the inducevoltage is in phase and has the same magnitude as the grid voltage. Inthis case, the induced voltages counter-act the grid voltage and, hence,stator winding draws no stator current. This is the system unity powerfactor case.

Note that a VAR/power factor control 670 may be incorporated into thesystem to control VAR production. (The product of the stator voltage Vsand the stator current vector Is (when no torque is produced) representsthe magnetizing VAR required by the generator.

Enable Turbine Operation

The power converter operates only when enabled. A turbine controllerenables and disables the power converter as shown in FIG. 7, Block 705.Such a turbine controller may be implemented in hardware, software, or acombination of both, such as in computer or controller based systems. Inone embodiment, the present invention uses binary logic voltage signalfor enabling and disabling the power converter, which is referred toherein as the converter enable signal

In one embodiment, when the turbine controller is in normal operationmode, referred to herein as auto mode, the turbine controller yaws theturbine into the wind and pitches the blades of the turbine to a fullpower position. The full power position would be well understood tothose skilled in the art. Given sufficient wind, the blades begin torotate and the generator speed accelerates. Once the generator speedreaches a preselected converter enable speed, the turbine controllersends the converter enable signal to the power converter. In oneembodiment, the preselected converter enable speed comprises 820 rpm.

In response to receiving the converter enable signal, a converterstartup sequence begins. In one embodiment, the system initially dosesthe AC line contactor (in inverter 630), which results in the linematrix (in inverter 630) being connected to the grid. A predetermineddelay allows this contactor to close and any transients to settle. Inone embodiment, this predetermined delay is a 1.5 second delay. Oneembodiment of the enable sequence is described in more detail inconjunction with FIG. 7, and blocks 714, 715, 716 and 717.

After the contactor is dosed, a bus precharge cycle occurs to ensurethat the bus is fully charged and to allow for regulating theinstantaneous torque. In this case, the DC bus voltage is regulated to apredetermined number of volts. In one embodiment, the predeterminednumber of volts comprises 750 volts DC. Another delay may be used toensure that the bus is precharged sufficiently to regulate properly. Inone embodiment, this delay may be 5 seconds. In one embodiment, if thebus fails to regulate, a bus over/under voltage fault is generated and aconverter fault is sent to the turbine controller.

When the generator speed reaches a preselected speed or above and thepredetermined bus delay has expired (i.e. after fully charging the busfor 5 seconds), the stator contactor is closed (block 714), therebyenergizing the stator windings and producing a rotating stator flux. Thestator windings are only energized with voltage. Due to the inductanceof the stator windings, the inrush current is very small, and in oneembodiment, only 75% of the maximum operating current. In oneembodiment, the preselected speed is 900 rpm. A delay may be used toallow the stator contactor to close and transients to settle. In oneembodiment, the delay is 3 seconds.

When the generator speed reaches a preselected speed or above, and therotor voltage is verified to be below a predetermined voltage peak, therotor contactor is dosed (block 715), connecting the generator matrix tothe rotor of the wound rotor induction generator. In one embodiment, thepreselected speed comprises 1000 rpm and the predetermined voltage peakis 318 volts. A delay may be used to allow the rotor contactor to close.In one embodiment, this delay is ½ second. Up to this time, the rotorside IGBTs (in inverter 624) are not switching. Since the rotor sideIGBTs are not yet switching, there is no current flow, and there is notransients or power production. Because there is not real power (onlyreactive power), no torque spikes are generated.

The production of power begins with the gating of the rotor side IGBTswhich creates the current vector both magnitude and position) requiredto produce the desired torque. In one embodiment, the current vector iscreated in response to a command from a torque controller (e.g.,processor). Initially, this torque is ramped up from 0 to the valuespecified by the optimal power/torque-speed curve. The ramp-up (block716) eliminates power or torque excursions and allows the turbine to bebrought on-line smoothly.

Note that the synchronization of the present invention is different thantraditional “synchronization” process used in synchronous or squirrelinduction machines; in the present invention, there is no inrush,transients or power oscillations associated with putting the turbineon-line.

Once synchronized, the power converter follows the power-speed curvedescribed above (block 717) until disabled by the turbine controller.

It should be noted that the delays discussed above with respect to theconverter startup sequence may be adjusted based on the components beingused in the system and the environmental conditions existing at theturbine site. These adjustments may be made in software, hardware, orboth.

In one embodiment, power into the turbine is provided by the wind. Ifthe wind speed changes, the turbine input power changes. To compensatefor changes in the input power, the present invention provides an updateprocess for updating the generator torque. Since the generator torque is(instantaneously) fixed by the power converter, the generator speedincreases in accordance with the power formula P=Tω. The powerconverter, which continuously samples generator speed, recognizes thatthe speed has changed and identifies the new speed, and updates thedesired power from the look-up table. The power converter determines anew torque from the desired power and, based on FOC, calculates a newcurrent vector which is impressed upon the generator rotor. In oneembodiment, the update process occurs every 33 milliseconds, or every 2cycles for a line at 60 Hz, causing the turbine to smoothly andaccurately follow the power-speed curve. Note that the update rate couldbe varied or could change dynamically during operation.

Below rated wind speed (e.g., Region II), the blades are kept at apreselected power capture angle, and the resulting generator/turbinespeed is due to the commanded torque and wind power input. This assuresthat the powered curve has been correctly chosen. In one embodiment, thepreselected power capture angle is the maximum power capture angle(e.g., 0, 1, or 2 degrees pitch). The number of degrees changes as afunction of the wind speed.

Rated power occurs at a predetermined generator rotor speed. In oneembodiment, the generator speed at which rated power occurs is 1423 rpm.Above rated wind speed, the generator rotor speed is controlled by thePID controller which pitches the blades in response to a generator rotorspeed indication. Note that this indication may be in a variety of formsthat include, but are not limited to, a signal or stored speed value(s)in a register. Importantly, the PID pitch controller works independentlyof the power converter. If the power converter fails, the PD controllermaintains the generator speed (1423 rpm in one embodiment) by commandinggreater blade pitch angles. By doing so, this system has a built-in failsafe operation.

For generator speeds equal to or greater than the generator speed atwhich rated power occurs (e.g., 1423 or more), the power-speed curve issuch that the power converter holds power constant, and withoutsignificant fluctuations. Hence, above rated speed wind gusts, whichtend to increase turbine speed, have little effect on generator power,as the PID controller responds and regulates generator rotor speed. Theresponse of the PID controller, however, is such that it is able toeffectively control rotor speed and thus power excursions to withinapproximately 5 percent, yielding a nearly flat power production forwind speeds equal to or greater than rated.

Above rated power excursions have no effect on grid voltage as excesspower is developed by the rotor of the wound rotor induction generatorsince stator power remains constant Rotor current (and stator current)is held constant during these excursions by the power converter byholding torque constant (torque being proportional to current). Sincerotor current is constant during these rusts, the increase in rotorpower is due to an increase in rotor voltage. But the grid is notaffected by this voltage increase because the power converter, situatedbetween the generator rotor and the grid, electronically translates thisvarying rotor voltage (and frequency) to a constant AC waveform (e.g.,60 cycle 460 volt AC waveform).

Full Span Variable Pitch Control System

The variable pitch control system (VPC) of the present invention is areal time, distributed, servo system for pitch position and rotor speedcontrol of the wind turbine. The VPC monitors and controls blade pitchposition, pitch velocity, and generator rotational speed.

In one embodiment, a pitch position transducer provides an analog signalthat is proportional to the blade pitch position, and later converted todigital, to identify the current position of the turbine blades. A bladeactuator coupled to the blades is used to mechanically change the pitchof the blades.

FIG. 7 is a flow diagram illustrating one embodiment of the pitchregulation system of the present invention. Control or processing logicin the system performs some of the operations in cooperation with theelectrical/mechanical hardware in the system. The control/processinglogic may be implemented in hardware, software, or a combination ofboth, such as in a computer or controller system.

Referring to FIG. 7, the pitch regulation system begins by measuring therotor speed (block 701). At the same time, the system determines itsoperational status (block 702). A test determines whether the pitchregulation system is in auto mode (block 703). If the operation statusof the system is not auto mode, a test determines if the generator rotorspeed (in rpm) is less than a predetermined speed (block 704). In oneembodiment, the predetermined speed is 1035 rpm. If system is not inauto mode and the generator rotor speed is less an the predeterminedspeed, the power converter is signaled to enter a disable sequence(processing block 705); otherwise, the system remains in its currentstate.

If the system is operating in auto mode, processing continues at block706 where a test determines if the generator rotor speed is increasing.If the generator rotor speed is not increasing, a test determines if thegenerator rotor speed is less than a predetermined set point (block707). In one embodiment, this predetermined set point is 835 rpm. If thegenerator rotor speed is not increasing and is less than 835 rpm, thepower converter is signaled to enter a disable sequence (block 705);otherwise, the system remains in its current state.

In one embodiment, the disable sequence comprises ramping the rotorcurrent down (block 708), disconnecting the rotor of the generator(block 709), and disconnecting the stator of the generator (block 710).

If the generator rotor speed is increasing as determined at block 706, atest determines whether the generator rotor speed is greater than 100rpm (block 711). If the generator rotor speed is greater than 100 rpm,the pitch is set to a predetermined set point (processing block 713). Inone embodiment, the predetermined set point is zero degrees. In otherembodiments, the pitch may set to any number of degrees, or portionsthereof, including one, two, or three degrees. In one embodiment, thepredetermined set point is variable.

Also, if the generator rotor speed is greater than 100 rpm, a testdetermines whether the generator rotor speed is greater than apredetermined speed (block 712). In one embodiment, this predeterminedspeed is 820 rpm. If the generator rotor speed is greater than thispredetermined speed, the converter is signaled to enter an enablesequence (processing block 705). Therefore, in this embodiment, thepower converter is enabled when the generator rotor speed is greaterthan 820 rpm.

In one embodiment, the enable sequence comprises the following steps.First, the generator-stator is connected to the grid (block 714). Afterconnecting the generator stator, the generator rotor is connected (block715). After connecting the generator rotor, the flux component of thegenerator rotor current, Ird, is ramped up (block 716) and then thegenerator torque is regulated (block 717). This enable sequence is apassive synchronization technique connecting the generator so as to comeon-line with the rotor current at zero. This is possible with the vectorcontrol in cooperation with the wound rotor induction generator of thepresent invention.

If the test determines that the generator rotor speed is increasing butis not yet over 100 rpm (block 711), the pitch is set to a predeterminednumber of degrees (block 718). In one embodiment, the pitch is set to 25degrees. Note that this pitch is a set point that may be varied. Thepitch should be chosen to obtain extra lift to help speed up the turbinefaster.

The present invention also performs the pitch position portion of thesystem. At first, the pitch position is measured, using well-knownmeasuring device (block 720). After measuring the pitch position, thepitch position error between the actual pitch and a predetermined setpitch is calculated (block 721).

After calculating the pitch position error, the pitch position error isamplified (block 722). With the amplified pitch position error and themeasured speed (block 701), the change in dynamic pitch rate is limited(block 723).

After limiting the dynamic pitch rate to a predetermined amount, a testdetermines whether the generator rotor speed is greater than apredetermined speed. In one embodiment, this set point is 1423 rpm. Ifthe generator speed is not greater than the predetermined speed, thepitch regulation system enters the fixed pitch position mode (block726); otherwise, the pitch regulation system enters the RPM regulationmode (block 727).

Pitch Regulation Mode

As discussed herein, pitch regulation refers to holding the blade pitchangle at the design operating position for operation below rated power.In one embodiment, this position is at 0 degrees. However, otherpositions may be employed. The VPC performs pitch regulation bycommanding a negative voltage that causes the pitch cylinder to movefrom its initial stop (e.g., 90 degrees) or feathered position at aconstant velocity of some number of degrees (e.g., 1.0) per secondtoward its nominal zero degree set point.

In the present invention, a position command voltage is applied to anerror amplifier to produce an error output that is proportional to thedifference between the command position (Pc) and the feedback position(Pf). In one embodiment, the error amplifier is software generated.However, such an amplification could be performed in hardware.

The output error is amplified and sent to the proportional valve. Aposition rate limiter is used to limit the pitch rate initially to onedegree per second. This limits the acceleration of the rotor in both lowand high winds and allows a smooth transition to generation withoutoverspeed problems.

Once the turbine has reached its zero degree position, the proportionalamplifier helps maintain this position by generating a voltage that isproportional to any error that would incur due to bleed down of thehydraulic system pressures. If, during initial pitching to the operatingpitch angle, the generator speed does not exceed a predetermined speed(e.g., 100 rpm), then the system pitches the blades to a predeterminedvalue (e.g., 25 degrees). This helps start the rotor tuning in verylight winds. Once the generator speed is above the predetermined speed,then the system pitches the blades to a nominal zero degree position.

Pitch regulation occurs at and above rated power (ie., in Region II)when the speed of the generator speed is below its rated set point(e.g., 1423 rpm). In one embodiment, during transitions from below ratedto above rated, the PD system begins to pitch the blades toward featherprior to the generator speed reaching the rated set point (e.g., 1423rpm) depending upon the acceleration of the generator rotor speed signal(from, for instance, block 607).

Pitch regulation below rated power does not require a full PID systemdue to the change of the pitch velocity being limited to only one degreeper second.

FIG. 8 illustrates one embodiment of the pitch position mode of thepresent invention. Referring to FIG. 8, the pitch position error value,which is proportional to the difference between the command position(Pc) and the feedback position (Pf), is calculated (block 800). Then atest determines whether the pitch error is positive (block 801). If thepitch error is not positive, then a test determines whether the rotorspeed is greater than a first predetermined speed set point (block 803).In one embodiment, the predetermined speed set point is 1200 rpm asmeasured at block 802. If the pitch error is not positive and thegenerator rotor speed is not greater than the first predetermined speedset point, processing continues at block 804 where the pitch rate limitis set equal to −Y1 and is input to the dynamic pitch rate limiter 805.

If the rotor speed is greater than the first predetermined speed setpoint, then a test determines whether the rotor speed is greater than asecond higher predetermined speed set point (block 806). In oneembodiment, the second predetermined speed set point is 1250 rpm. If therotor speed is greater an the second predetermined speed set point, thenprocessing continues at block 807 where the pitch rate value Y is set to−Y2 and is input to the dynamic pitch rate limiter 805. If the rotorspeed is not greater than second predetermined speed set point, then thepitch rate limit value Y is set to a function of the rotor speed (block808), which is between −Y1 and −Y2, and the pitch rate limit value Y issent to the dynamic pitch rate limiter (block 805). In one embodiment,this function is a linear function of the pitch rate limiter that rampsbetween a minimum and a maximum.

If the pitch error is positive, then a test determines whether the rotorspeed is greater than a third predetermined speed set point (block 809).In one embodiment, the third predetermined speed set point is 1100 rpm.If the pitch error is positive and the generator rotor speed is notgreater than he third predetermined speed set point, processingcontinues at block 810 where the pitch rate limit Y is set equal to Y1and is input to the dynamic pitch rate limiter (block 805).

If the rotor speed is greater than the third predetermined speed setpoint, then a test determines whether the rotor speed is greater than afourth predetermined speed set point (block 811). In one embodiment, thefourth predetermined speed set point is 1150 rpm. If the rotor speed isgreater than the fourth predetermined speed set point, then processingcontinues at block 812 where the pitch rate limit value Y is set to Y2and is input to the dynamic pitch rate limiter (block 805). If the rotorspeed is not greater than the fourth predetermined speed set point, thenthe pitch rate limit value Y is set to a function of the rotor speed(block 813), which is between Y1 and Y2, and the pitch rate limit valueY is sent to the dynamic pitch rate limiter (block 805). Thus, thefunction is in the opposite direction of the function of block, 808described above. In one embodiment, this function is a linear functionof the pitch rate limiter that ramps between Y₁ and Y₂, a maximum and aminimum, respectively.

The pitch position error value determined at block 800 is amplified(block 814) and input to the dynamic pitch rate limiter (block 805). Inresponse to the pitch rate limit value Y and the amplified pitchposition error value, the pitch rate change is limited initially to onedegree per second to limit acceleration of the rotor in both low andhigh winds and to allow a smooth transition to generation without overspeed problems.

A test determines whether the measured rotor speed from block 802 isgreater than a fifth predetermined speed set point (block 815). In oneembodiment, the fifth predetermined speed set point is 1423 rpm. If themeasured rotor speed is greater than the fifth predetermined speed setpoint, the system enters the RPM regulation mode (block 816). On theother hand, if the measured rotor speed is not greater than the fifthpredetermined speed set point, then the pitch rate is set to aprogrammed value (block 817), which may be represented as a binaryvoltage, and processing continues at block 818.

At block 818, a test determines whether the system is in auto mode. Inone embodiment, this test is determined by examining whether the systemis in stand by/stop fault mode as a result of a fault being detected atblock 819. If the system is not in auto mode, processing continues atblock 820 where the pitch control is overridden to turn off the system.In one embodiment, the system is turned off by pitching the blades to90°. If the system is in auto mode, then the binary voltage representingthe programmed values is converted to analog (block 821) and drives ahydraulic system proportional valve (block 822).

In one embodiment, a single digital-to-analog converter (D/A) generatesthe voltage required by the hydraulic proportional valve. This voltageis directly proportional to the velocity of the hydraulic pitchcylinder, i.e., the rate of change of blade pitch position. In oneembodiment, a positive voltage causes the blades to pitch toward thefeather direction (pitch to feather), while a negative voltage causesthe blades to pitch toward the power direction (pitch to power). Thepitch rate is controlled by the amplitude of the D/A output voltage. Inone embodiment, an output sample rate of the D/A is fixed at 10 Hz.

RPM Regulation Mode

The VPC system regulates generator speed. In one embodiment, generatorspeed is regulated by a Proportional, Integral and Derivative (PID)control of the turbine blade pitch angle. The VPC system calculates andthen later amplifies an error, via software in one embodiment, toproduce an output error that is proportional to the difference betweenthe commanded speed (e.g., 1423 rpm), which is referred to herein as Rc,and the feedback speed, referred to herein as Rf. The present inventionuses his output to generate PID values required for correct velocitycontrol of the proportional valve and, hence, the blade pitch angle.

When the rotor speed nears a predetermined set point (e.g., 1423 rpm),the PID controller generates a voltage that pitches the blades towardfeather. Conversely, when the rotor speed drops below the predeterminedset point (e.g., 1423 rpm), the PID controller generates a voltage thatpitches the blades toward power until again reaching the nominal pitchsetting or exceeding the nominal predetermined set point (e.g., 1423rpm).

The PID speed regulation controller is a velocity based system. In oneembodiment, a table is used to change the pitch rate values generated bythe PID control logic into specific voltages to be applied to theproportional value. An example table is shown in Table 2. In oneembodiment, the maximum pitch to feather velocity is 12 degrees persecond while the maximum pitch to power velocity (during speedregulation) is 8 degrees per second. These correspond to output D/Avoltages of 5.1 and 4.1, respectively.

TABLE 2 Pitch Rate to Drive Voltage Translation Table RATE VOLTAGEdeg/sec −8.25 −20 −7.90 −19 −7.55 −18 −7.20 −17 −6.85 −16 −6.50 −15−6.15 −14 −5.80 −13 −5.45 −12 −5.10 −11 −4.75 −10 −4.40 −09 −4.05 −08−3.41 −07 −3.12 −06 −2.88 −05 −2.67 −04 −2.34 −03 01.96 −02 −1.45 −010.00 00 1.83 01 2.33 02 2.71 03 3.12 04 3.46 05 3.79 06 4.08 07 4.25 084.45 09 4.65 10 4.85 11 5.05 12 5.25 13 5.45 14 5.65 15 5.85 16 6.05 176.25 18 6.45 19 6.65 20

Note that in Table 2, a negative pitch rate is a pitch to power, while azero or position pitch rate is a pitch to feather.

In one embodiment, a valve control switch turns off the proportionalvalve during Stop and Standby modes as commanded.

FIG. 9 illustrates one embodiment of the rpm regulation mode of thepresent invention. Referring to FIG. 9, at block 900, the speed errorvalue that is proportional to the difference between the commanded rpm(Pc) from (block 930) and the measured rpm (Pf) from block 902 iscalculated (block 900).

A test determines whether the rpm error is positive (block 901). If thespeed error is not positive, then a test determines whether the rotorspeed is greater than a first predetermined speed set point (block 903).In one embodiment, the predetermined speed set point is 1200 rpm. If therpm error is not positive and the generator rotor speed is not greaterthan the first predetermined speed set point, processing continues atblock 904 where the pitch rate limit value is set equal to −Y1 and issent to the dynamic pitch rate limiter 905.

If the rotor speed is greater than the first predetermined speed setpoint, then a test determines whether the rotor speed is greater than asecond higher predetermined speed set point (block 906). In oneembodiment, the second predetermined speed set point is 1250 rpm. If therotor speed is greater than the second predetermined speed set point,then processing continues at block 907 where the pitch rate limit valueY is set to −Y2 and is input to the dynamic pitch rate limiter 905.

If the rotor speed is not greater than second predetermined speed setpoint, then the pitch rate limit value Y is set to a function of therotor speed (block 908). In one embodiment, this function is a linearfunction of the pitch rate limiter that ramps between −Y1 and −Y2. Thepitch rate value Y is sent to the dynamic pitch rate limiter (block905).

If the speed error is positive, then the pitch rate limit value Y is setto Y2 (block 912) and is input to the dynamic pitch rate limiter (block905).

Also after calculating the speed error value, the PID system determinesif the acceleration is too fast and sets the pitch accordingly (block940). In response to the pitch rate limit value Y and the output of thePID loop 940, the pitch rate is limited to initially to one degree persecond (block 905).

Then a test determines whether the measured rotor speed (block 902) isgreater than a third predetermined speed set point (block 915). In oneembodiment, the third predetermined speed set point is 1423 rpm. If themeasured rotor speed is less than the third predetermined speed setpoint, the system enters the pitch position mode (block 916). On theother hand, if the measured rotor speed is greater than the thirdpredetermined speed set point, the pitch rate is converted using thepitch rate to drive voltage translation table described above (block917), and processing continues at block 918.

At block 918, a test determines whether the system is in auto mode. Inone embodiment, this test is determined by examining whether the systemis in stand by/stop fault mode as a result of a fault being detected atblock 919. If the system is not in auto mode, processing continues atblock 920 where the pitch control is overridden to turn off the system.In one embodiment, the system is turned off by pitching the blades to90°. If the system is in auto mode, then the voltage representing thepitch rate value is converted to analog (block 921) and is applied tothe hydraulic system proportional valve to initiate pitching action(block 922).

A Pitch System with a PID Controller

FIG. 10A illustrates one embodiment of one pitch system. Referring toFIG. 10A, the pitch system comprises a closed loop PID controller 1010and a non-linear table 1011 for converting pitch velocity inputs tovoltage outputs. The pitch velocity values received by table 1011 aregenerated by PD controller 1010 in response to a difference in outputspeed and commanded speed, as determined by comparison logic orsoftware. The voltage outputs from table 1011 are applied to aproportional value, which results in blade pitch action.

A block diagram of the functional flow of one embodiment of the PIDcontroller is shown in FIG. 10B. Referring to FIG. 10B, a difference isdetermined between the position feedback value, Pf, from the positioncommanded, Pc by comparison logic (e.g., a subtractor) or software 1001.This difference represents the position error. The position error isamplified by a scale factor of K by amplifier 1002 to create the valueyc. In one embodiment, K is set at 0.5. The value yc is coupled as aninput to limiter 1005, which is controlled by limiter controller 1004.Limiter 1005 limits the pitch rate of the blades during pitch positionmovements. In one embodiment, the pitch rate is slow. Controller 1004 iscoupled to receive the generator speed feedback and, in responsethereto, changes the limiter 1005 based on the generator speed (in rpm).The limiter controller (block 1004) ramps maximum pitch to feather orpitch to power rate using a linear function of measured value of rpm,R_(F).

The PID controller also comprises comparison logic (e.g., a subtractor)or software 1003 to generate a difference between the commandedgenerator speed, Rc, and the actual generator speed, Rf. The output ofcomparison block 1003 is the speed error value x, which is received bythe PD algorithm blocks 1006 and 1007. The PID algorithm (blocks 1006and 1007) compute a desired pitch rate based on a proportional, integraland derivative function of the speed error value. The pitch rate outputas a function of speed error input may also include gain scheduling thatadjusts gains as a function of pitch position. A gain rate scheduler(block 1012) provides the multiplier, E, based on pitch positionfeedback and two set point parameters E1 and E2. In one embodiment, thetwo set point parameters E1 and E2 are=−0.85 and 0.0028 respectively.The output of the block 1005 is coupled to the output of 1006 and yf tofeed into block 1008. Limiter 1005 limits the maximum pitch velocity ofpitch to feather and pitch to power during speed regulation mode.

The output of limiter 1008 provides the input of a voltage generator1009 and feeds back into PID algorithm block 1007. The output of voltagegenerator 1009 is coupled to the input of switch 1010 which iscontrolled to shut off the proportional value in response to a commandto stop the turbine. The output of switch 1010 is coupled to a D/Aconverter 1011 that provides the voltage output for the system that isapplied to the proportional value driving the blade pitch action.

Dynamic Braking

To achieve dynamic braking, the torque-speed curve of the presentinvention may be deliberately skewed. In one embodiment, the powerconverter commands a maximum constant torque. This maximum constanttorque is switched into the system in response to a fault condition,causing the turbine speed to decrease. FIG. 6B illustrates the powerconverter including a maximum constant torque 660 and switch 629.

In one embodiment, the safety system initially applies a soft brake andpitches the blades to 90 degrees. Afterwards, a test determines whetherthere has been a fault. In one embodiment, dynamic braking is only usedin response to hard stop faults. In other embodiments, dynamic brakingmay be used for other types of faults (e.g., soft, hard, etc.).

In response to determining that a hard stop fault occurred, the presentinvention pitches the blades to 90 degrees and commands the maximumconstant torque value. The torque is impressed upon the generator rotor,resulting in a decrease in turbine speed. In one embodiment, the turbineis slowed to a predefined speed. After attaining the predefined speed,the braking may be released, either automatically or manually (e.g.,manual reset by operator).

Power Factor and VAR Compensation

Since the power converter controls the rotor current directly, the totalsystem power factor can be controlled and adjusted dynamically over arange of 0.90 lagging to 0.90 leading regardless of turbine output levelIn the present invention, the VARs are supplied to the secondary of theinduction generator. Thus, the power converter can act as a VARcompensator for the utility. This is accomplished by a control systemwhich commands a specific number of KVARs from each turbine through aSCADA system. FIG. 6B illustrates an input 670 to control the VARs. Byadjusting the supply of VARs to the secondary, total system VARs can beselected dynamically.

The desired power factor can be set at any nominal value between 0.90lagging and 0.90 leading or vary in response to fluctuations in gridvoltage. Hence, the power converter, working through SCADA can operatein a constant power factor mode, constant VAR mode, or a voltageregulating mode.

Some of the benefits of the power conditioning of the present inventionis that it provides maximal energy capture, torque control, eliminationof voltage flicker, as well as power factor control. In addition,dynamic power factor adjustment is available. Furthermore, the variablespeed of the present invention provides for mitigating torque spikes.Torque transients, which cause voltage flicker and damage to the drivetrain components, are attenuated by allowing an increase in rotor speed,thereby “storing” the additional energy of a wind gust in a rotationinertia of the rotor blades. This energy can be extracted and fed intothe grid by reducing the rotor speed as the wind gust dies or it can be“dumped” by pitching the blades out of the wind. Thus, variable speedoperation can dramatically reduce torque transients which translates tolower cost and longer life for the wind turbine drive train.

Some portions of the detailed descriptions described above are presentedin terms of algorithms and symbolic representations of operations ondata bits within a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent sequence of steps leading to a desiredresult. The steps are those requiring physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present invention,discussions utilizing terms such as “processing” or “computing” or“calculating” or “determining” or “displaying” or the like, may refer tothe action and processes of a computer system, or similar electroniccomputing device, that manipulates and transforms data represented asphysical (electronic) quantities within the computer system's registersand memories into other data similarly represented as physicalquantities within the computer system memories or registers or othersuch information storage, transmission or display devices.

Also as discussed above, the present invention also relates to apparatusfor performing the operations herein. This apparatus may be speciallyconstructed for the required purposes, or it may comprise a generalpurpose computer selectively activated or reconfigured by a computerprogram stored in the computer. Such a computer program may be stored ina computer readable storage medium, such as, but is not limited to, anytype of disk including floppy disks, optical disks, CD-ROM;, andmagneto-optical disks, read-only memories (ROMs), random access memories(RAMs), EPROMs, PROMS, magnet or optical cards, or any type of mediasuitable for storing electronic instructions, and each coupled to acomputer system bus. The algorithms presented herein are not inherentlyrelated to any particular computer or other apparatus. Various generalpurpose machines may be used with programs in accordance with theteachings herein, or it may prove convenient to construct morespecialized apparatus to perform the required method steps. The requiredstructure for a variety of these machines will appear from thedescription below. In addition, the present invention is not describedwith reference to any particular programming language. It will beappreciated that a variety of programming languages may be used toimplement the teachings of the invention as described herein.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that theparticular embodiment shown and described by way of illustration is inno way intended to be considered limiting. Therefore, references todetails of the various embodiment are not intended to limit the scope ofthe claims which in themselves recite only those features regarded asessential to the invention.

Thus, a variable speed system has been described.

We claim:
 1. A synchronization process for a variable speed systemhaving a generator, said process comprising: connecting a generatorstator at a first generator speed; connecting a generator rotor at asecond generator speed higher than the first generator speed when rotorvoltage is at a first voltage; ramping up a rotor current; andregulating generator torque.
 2. The process defined in claim 1 whereinregulating generator torque comprises enabling a rotor side converterand gating rotor side IGBTs.
 3. The process defined in claim 1 whereinregulating generator torque comprises creating a current vector that isable to produce the desired torque.
 4. The process defined in claim 1wherein the generator comprises a wound rotor induction generator. 5.The process defined in claim 1 wherein the generator comprises a doublyfed generator.
 6. An apparatus for a variable speed system having agenerator, said apparatus comprising: means for connecting a generatorstator at a first generator speed; means for connecting a generatorrotor at a second generator speed higher than the first generator speedand when rotor voltage is at a first voltage; means for ramping up arotor current; and means for regulating generator torque.
 7. Theapparatus defined in claim 6, wherein means for regulating generatortorque comprises means for enabling a rotor side converter and means forgating rotor side IGBTs.
 8. The apparatus defined in claim 6 whereinmeans for regulating generator torque comprises means for creating acurrent vector that is able to produce the desired torque.
 9. Theapparatus defined in claim 6, wherein the generator comprises a woundrotor induction generator.
 10. The apparatus defined in claim 6, whereinthe generator comprises a doubly fed generator.
 11. A system comprising:a wound rotor induction generator having a generator stator and agenerator rotor; and a torque controller coupled to the generator toregulate generator torque, wherein the generator stator operates at afirst generator speed, wherein the generator rotor operates at a secondgenerator speed higher than the first generator speed when rotor voltageis at a first voltage, and wherein a rotor current ramped up.
 12. Thesystem defined in claim 11, wherein the generator torque is regulated byenabling a rotor side converter and gating rotor side IGBTs.
 13. Thesystem defined in claim 11, wherein the generator torque is regulated bygenerating a current vector to produce a desired torque.
 14. The systemdefined in claim 11, further comprising a doubly fed generator.
 15. Asystem comprising: a wound rotor induction generator having a generatorstator and a generator rotor; a torque controller coupled to thegenerator to control generator torque using field oriented control; anda pitch controller coupled to the generator to perform pitch regulationbased on generator rotor speed and operating independently of the torquecontroller, wherein the generator stator operates at a first generatorspeed, wherein the generator rotor operates at a second generator speedhigher than the first generator speed when rotor voltage is at a firstvoltage, and wherein a rotor current ramped up.
 16. The system definedin claim 15 wherein the pitch controller comprises a proportional,integral derivative (PID) pitch controller.
 17. The system defined inclaim 15, wherein the pitch controller comprises a proportional,integral (PI) pitch controller.
 18. The system defined in claim 15,wherein the pitch controller comprises a proportional, derivative (PD)pitch controller.
 19. The system defined in claim 15, wherein the pitchcontroller comprises a Lag-Lead controller.
 20. The system defined inclaim 15, wherein the pitch controller comprises a Lead-Lag controller.21. The system defined in claim 15, where the pitch controller comprisesan open loop controller with a derivative term.
 22. The system definedin claim 15, wherein the wound rotor induction generator comprises anon-slip ring induction generator.
 23. The system defined in claim 15,wherein the torque controller comprises a dampening filter to reducecommanded torque based on detected oscillation motion between turbineblades and the generator.
 24. A system comprising: a wound rotorinduction generator having a generator stator and a generator rotor; atorque controller coupled to the generator to control generator torqueusing field oriented control; and a proportional, integral derivative(PD) pitch controller coupled to the generator to perform pitchregulation based on generator rotor speed, wherein the generator statoroperates at a first generator speed, wherein the generator rotoroperates at a second generator speed higher than the first generatorspeed when rotor voltage is at a first voltage, and wherein a rotorcurrent ramped up.
 25. The system defined in claim 24, wherein the woundrotor induction generator comprises a non-slip ring induction generator.26. The system defined in claim 24, wherein the power controllercontrols the generator power and torque as a function of generatorspeed.
 27. The system defined in claim 24, wherein the power controllercontrols the generator power from a power look up table (LUT) as afunction of generator speed using field oriented control (FOC).